Known long-haul systems have been implemented using binary modulation formats wherein a single data bit is modulated on a single transmitted symbol. One method of performing binary modulation is known as On-Off-Keying (OOK), wherein the transmitted pulse is turned on and off with the ones and zeros of a data bit stream. On-Off-Keying may be implemented in a variety of well-known formats, such as Return-to-Zero (RZ), Non-Return to Zero (NRZ) and Chirped-Return-to-Zero (CRZ) formats. Generally, in an RZ format the transmitted optical pulses do not occupy the entire bit period and return to zero between adjacent bits, whereas in a NRZ format the optical pulses have a constant value characteristic when consecutive binary ones are sent. In a chirped format, such as CRZ, a bit synchronous sinusoidal phase modulation is imparted to the transmitted pulses.
Differential phase-shift-keying (DPSK) is another binary modulation method known to those of ordinary skill in the art. In a DPSK format, the optical intensity of the signal may be held constant, while ones and zeros are indicated by differential phase transitions. Variations of the DPSK modulation formats include RZ-DPSK, wherein a return-to-zero amplitude modulation is imparted to a DPSK signal, and CRZ-DPSK.
When the bit rate of a transmission system is increased, e.g. to 40 Gb/s, transmission penalties may become more pronounced. For higher bit rates, multi-level modulation formats have been attractive due to their high spectral efficiency and increased tolerance to chromatic dispersion and polarization mode dispersion compared with the same line rate as binary modulation formats. In a multi-level modulation format multiple data bits may be encoded on a single transmitted symbol.
A number of multi-level modulation formats are known. Two types of multi-level modulation formats that have been considered for 40 Gb/s long-haul optical transmission systems are return-to-zero differential quadrature phase shift keying (RZ-DQPSK) and polarization multiplexing (POLMUX) RZ-DPSK, which involves sending a pair of 20 Gb/s signals at orthogonal polarizations. Both of these formats achieve 2 bits/symbol modulation.
It has been found that that 40 Gb/s POLMUX-RZ-DPSK outperforms 40 Gb/s RZ-DQPSK by ˜4 dB in a 5,200 km Raman assisted EDFA system with 150 Km repeater spacing and 66.6 GHz channel spacing mainly due to higher nonlinear phase noise tolerance and sensitivity. Unfortunately, however, POLMUX-RZ-DPSK requires a receiver including a polarization controller to track the input random state of polarization for each channel and a polarizer to suppress the orthogonal neighboring channel. Any loss of polarization control may result in a degradation of Q-factor in a POLMUX-RZ-DPSK system.
A frequency offset polarization division multiplexing (PDM) method and system was developed to eliminate the need for the polarization tracker at the receiver, as described in greater detail in U.S. patent application Ser. No. 12/187,084 filed Aug. 6, 2008, which is fully incorporated herein by reference. According to this frequency offset PDM technique, the frequencies of two polarization combined DPSK signals may be separated by ¼ of the free spectral range (FSR) of the demodulator.